The 35-year-old joined Blues in the summer as back-up to goalkeepers Jack Butland and Colin Doyle and made one appearance, starting the season-opening 5-1 Capital One Cup victory over Barnet.
But having spoken of his interest in a move into coaching, Lucas - formerly of Preston, Sheffield Wednesday and Swindon among others - has been cleared to make the switch to Fleetwood, with whom he will also be registered to continue his playing career.
"This was an opportunity too good for me to turn down," Lucas told Fleetwood's official website.
"I've had a taster with Graham Alexander and Chris Lucketti (Town boss and coach respectively) at Preston North End, working with their youth team and Academies on my day off, and it's something I have really enjoyed as I've got older.
"The chance to work with senior goalkeepers is something I've been itching to do, and to combine that with playing while I'm still fit and able is fantastic."